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Sometimes in science fiction, you'll find aliens whose bodies replace carbon with silicon. In hard science fiction, the plausibility of this trope rests on carbon and silicon being in the same family of elements: the ability to form four bonds opens the door to a vast array of potential complex nanostructures.

Silicon biochemistry would be wildly different though: you can't replace carbon with silicon in known biomolecules and get anything functional. Even the small "hydrosilicon" molecules are much more reactive than their carbon counterparts. Plus, carbon is actually not so common on the earth, with the SECOND most abundant element being... wait for it... silicon.note silicon dioxide is the most abundant compound on earth but as the name suggests, oxygen is at least twice as abundant as silicon. If that kind of life were able to form, it's reasonable to assume it would have done so here (as you know, it didn't).

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It's somewhat more chemically plausible to base life on silicates (silicon oxides), which have a huge variety in nature. These species would either have to exist at molten-glass temperatures, or be very slow (literally geological). Silicate based lifeforms would definitely be more Starfish Aliens than Rubber-Forehead Aliens.

Haruhi Suzumiya: Silicon-based Data lifeforms were responsible for the mysterious happenings in one of the two stories of the eighth volume. But they live on without their bodies. How? That's classified, apparently.

Blame! features a race called the "Silicon Creatures" or "Silicon Lives". Like the rest of the cast, they're actually just advanced cyborgs who "evolved" from humans, but they're so heavily altered that it's hard to say if the fleshy-looking parts of them are really flesh or not anymore.

In Project ARMS, the alien life form Azreal is silicon-based. The discovery of it is what gets the Egrigori to kick-start their various ARMS experiments. It also saves the day at the end, when it's revealed that Azreal is nearly immortal because of its silicon makeup, which leaves it very lonely and more willing to sympathize with the protagonists than Keith White (who wants to kill everyone on Earth and essentially condemn Azreal to an eternity alone).

In another DC Universe story by Alan Moore collected in the same book, there is a rare example of a more realistic portrayal of silicon lives as they would exist at Earth-like temperatures. When their planet is invaded by the power-hungry Spider Guild, the silicon beings, resembling giant stone statues of men, don't even notice because they move at a geologic pace.

Marvel's most frequently seen silicon-based aliens are the Kronans, introduced in the very first adventure of The Mighty Thor as the "Stone Men From Saturn," though it was later established that they'd only maintained a temporary base there.The Incredible Hulk's buddy Korg is their most well-known representative.

Though they've never been officially declared to be the same race, Jack Kirby did a story in one of Marvel's pre-Fantastic Four giant monster titles where the stone heads on Easter Island turn out to be stone aliens buried up to their necks, and he used the same design that he would later use for the Kronans in Thor.

Fanfic

Stargate Atlantis: In the Dark Frontier fanfiction, the Shade are a strange hybrid of carbon- and silicon-based life: they are mostly carbon-based but use a powerful acid to digest any mineral in order to build and repair a silicon-based exoskeleton. Their unusual composition also gives them the ability to use ammonia and sulfuric acid for what we use water. Oh, and they are cold-blooded, capable of draining anything from alkaline batteries to energy shields in order to stay alive for months without feeding in any environment. The biggest ones (few thousand miles across) strip mine entire planets for naquadah and trinium, also draining the planetary core for heat; when they move on, the planet is essentially a dead husk locked in an eternal ice age while the metals are used to build planetoids which serve as hatcheries. Despite being living creatures, they are apparently capable of entering hyperspace.

All the World's a Stage has the Ania as such, being described as made of glass. Alex Rand says they're microorganisms taking form upon contact with any solids, both crystalline and amorphous. He also describes then as dangerous predators, able to rip apart anyone in mere seconds. By the way they're described and how they act, they bear a strong resemblance to the Broken Glass Creature from a Touch of Glass, a Creepypasta.

The giant space slug in Star Wars V , which is officially called an exogorth, is silicon-based, as are the winged creatures inside it, which are called mynocks. This detail is not mentioned in the film and comes from the Expanded Universe.

The titular Smog Monster from Godzilla vs. Hedorah is mentioned in passing to be an alien mineral-based lifeform, comprised of a form of crystallized carbon that bonds with waste particles and fluids to become an animate sludge.

Max Steel has Steel, a techno-organic life forms that feeds off energy, made of silicon. Another way to say it is an alien robot.

Sentenced to Prism, a Humanx Commonwealth novel by Alan Dean Foster. Most life on Prism is silicon based. Foster has a lot of fun with the possibilities: natural Frickin' Laser Beams, rich colouration, lots of armour, a casual approach to being on land or underwater (the inhabitants consider water 'thick air') and so forth.

Stanley G. Weinbaum's short story "A Martian Odyssey" had a silicon-based life form that consumed silicon and excreted silicon dioxide bricks.

Discworld: Played for laughs, but Trolls qualify. It is specifically noted that, as silicon-based organisms, they function like supercomputers, with processing speed (and thus intelligence) being inversely proportional to temperature. One of them uses Hulk Speak at room temperatures, but when locked in a freezer for a while, almost came up with a unified field theory. They also count in either base four or binary because of that.

The third Young Wizards book has a planet of silicon-based life forms, the fact of which ends up being extremely important. As it turns out, they can function like extremely complex computers.

Later books mention in passing that to some silicon-based species, chocolate is an aphrodisiac.

The Dancing Meteorite by Anne Mason includes a flashback to humans' first encounter with silicon-based life, which the human explorers think is just part of the landscape until it rises up and attacks them.

Ullerans in H. Beam Piper's Uller Uprising. The concept is made more believable by using siloxanes (alternating silicon-oxygen chains), some of which are pliable at what we consider room temperature. Piper didn't come up with idea - he was presented with an introductory essay by Dr. John D. Clark describing life on both Uller and Niflheim (see entry below under "Other Elements".

Dark is the Sun by Philip José Farmer: Phremompit is a silicon based lifeform native to an asteroid, coming to Earth in a meteor shower. He eats radioactive rocks and moves on natural treads. Unfortunately, he drills through many people before learning his morse-code communication laser is turned up a bit too strong for the mushy-bodied earthlings.

In Ben Bova's Venus, it's discovered that such a lifeform inhabits the titular planet. It has a massive underground body, and tentacles a few kilometers long which it uses to search the planet's surface, for food or whatnot.

The vampiric beings from The Stress of Her Regard appear to be this trope, although it's slightly too early in history for the characters to describe them in terms of modern chemistry.

In The Laundry Files, there are creatures living in the Earth's mantle—codeword DEEP SEVEN, or "Cthonians". True to the more realistic version of this trope, they freeze to death at any temperature where rock is a solid. True to the general tone of the Laundry Files, they're massively more technologically advanced and numerous than us, have insanely powerful war drones that are fully capable of coming up to the surface to squash us, and may realize any day now that they have very good reason to want us squashed.note They even scare the crap out of BLUE HADES, who also meet the description in the previous sentence exactly.

In The Osmerian Conflict, one of the alien species are the Silicians. As their name suggests, the major element in their bodies is silicon in the form of silicates.

Years earlier, the tie-in book Worlds of the Federation depicted the Tholians as little more than floating crystals with eyes, so essentially they were just heads, but Enterprise rendered this non-canon.

The Excalbians from "The Savage Curtain" are also heat-lovers, but they can survive at lower temperatures because they are much smarter and more powerful than the Tholians, or humans for that matter.

The extinct Martian insects that are sometimes displayed on computer screens were, according to certain reference books, also silicon-based lifeforms.

The "Crystalline Entity" that wiped out all life on the planet Data was found on.

Star Trek: Deep Space Nine "The Alternate" The protagonists find a silicon-based, shapeshifting organism while exploring L-S VI in the Gamma Quadrant. The organism is said to have a similar biology to Odo and there are structures on the planet that are near-identical to those on the Changeling homeworld, suggesting some relationship between it and the Changelings but it is never expanded on.

Star Trek: Enterprise "Observer Effect": Sato and Tucker are infected with a silicon based virus while excavating a Klingon garbage site. In a case of No Biochemical Barriers, the alien virus was not only very effective at infecting carbon based life-forms despite the different chemistry but it also had infected and killed Cardassians and Klingons.

The X-Files: The creature from "Firewalker" was a silicon based fungus-like creature that exploded from people's throats to spread its spore.

Doctor Who has several examples, including the Kastrians from "The Hand of Fear" and the Ogri from "The Stones of Blood".

Disneyland: "Mars And Beyond" briefly mentions silicon based life in a segment about what life on Mars would look like. It suggests that it would take the form of crystalline structures that rise during the day, then shatter into oblivion in the cold Martian night.

Although no actual life form appears, Sheldon of The Big Bang Theory builds a model of a hypothetical silicon-based DNA analogue.

Carl Sagan explored this question in the fifth episode of Cosmos: "I wonder if we'll ever find a specimen of life based not on organic molecules, but on something else — something more exotic." He goes on to describe himself as a "carbon chauvinist."

Classic Journal of the Travellers' Aid Society #15 article "The Bestiary". Doyle's eel is a silicon-based life form that eats metal. If it infiltrates a starship it will try to eat metals, silicon and some plastics, which can cause serious damage. It will also try to lay eggs and hatch out more of them, which will cause even more damage.

Marc Miller's Traveller supplement Milieu 0 Campaign, chapter "The First Wave". In one of the Survey adventures, the PCs can encounter large rock-like creatures that are silicon life-forms. They are serpent-like and can project lava jets as a weapon.

Towsers in Myriad Song may look like oddly colored dogs, but their bodies are composed of silicon and rare earths. They eat polysiloxanes, are immune to metal-based poisons, and can get oxygen from carbon monoxide (carbon dioxide is still deadly though).

Star Frontiers adventure SF1 Volturnus, Planet of Mystery. The Eorna created silicon-based life in the form of large crystals. Their attempts to make the crystals intelligent failed, as whenever they reached semi-intelligence they Turned Against Their Masters.

Chaosium's supplement All the Worlds' Monsters Volume III. The Zend-Avesta is a telepathic silicon based life form that gets its energy from sunlight and looks like a spherical white globe hovering in the air.

Lords of Creation RPG, Heroes magazine Volume 1 #1 article "Survival Run of the Starnomads". The flamewolf is an alien creature resembling large wolves made out of rock. They are silicon-based life forms with saliva and blood that burst into flames in the presence of oxygen.

Ares magazine #6 game "Voyage of the BSM Pandora". One of the possible encounters in the game is a huge silicon-based monster which is powerful enough to pulverize one of the expedition's robots.

GURPS Space Bestiary edition has Skylens. Predators that are transparent, silicon disks about three feet in diameter, attacking their targets with sun rays. They locomote by psychokinesis.

The Taalo in the games' backstory as well, though they were living rocks rather than specifically crystals. Their unique physiology is what enabled them to make friends with the Ur-Quan (who, being predators, usually feel an overwhelming instinct to eat other creatures they come into contact with... but rocks don't provoke that instinct).

Master of Orion: The Silicoids. The wildly divergent biochemistry of silicon-based organisms is reflected in the gameplay. Silicoids do not need to farm to produce food, thus freeing up their population to conduct research or construction. In addition, they are immune to the effects of pollution and can live on even the harshest planets without a problem. On the down side, their slower metabolism means slower population growth, and their vastly different psychology makes it nearly impossible to deal with other races diplomatically.

Also, the Space Crystal monster, which flies around and exterminates the population of colonized planets.

Sword of the Stars has the Swarm, bee-like things that spawn from asteroid hives and fire plasma, as well as sending out Silicoid Queens to build new hives. They are Demonic Spiders, especially in early game or with small groups, but reward you with a fair amount of resources if you clear a hive.

Technically, most Rock-type Pokémon count, as several of them are literally made of rocks. Same goes for many Ground-type and Steel-Type Pokémon. Porygon is either this trope or an android. (It's hard to tell.)

The Silacoids from X-COM: UFO Defense are simple silicon-based life forms that look like lumps of purple lava. They have rock-hard skin that is immune to fire damage and attack by biting. Because of their extremely high body temperature, they leave burn marks on the ground as they move, sometimes setting fire to nearby objects.

In XCOM: Enemy Unknown, researchers are forced to admit that they have no idea whether the Cyberdisks are living or purely mechanical, but conclude that if they are living, then they're probably silicon-based lifeforms. The Sectopods, on the other hand, are alive, and are silicon-based.

Muv Luv: The BETA's creators are this, and believe that only Silicon Based Life is real life. To the degree that carbon based life isn't even considered alive. In other words, the BETA don't even think they're fighting a war. From their point of view they're recycling resources.

In Duke Nukem Forever, on the alien ship there are breasts growing out of the wall. Duke will make a quip if you slap them, one of which is "Hmm, some strange silicon(e) based life form."

An enemy in Borderlands 2 are "crystalisks", which are tripodal organisms made of living rock and crystal — the base of each leg features a large patch of golden crystal, their only vulnerable spot, and they drop over a dozen valuable crystal shards upon death. One quest, leading into a long-abandoned mine called the Caustic Caverns, revolves around recovering an Apocalyptic Log that reveals that the crystalisks are actually highly intelligent and were friendly when humans first arrived. The security forces for the first mining expedition even made friends with them. When the profit-orientated mining officer demanded that the crystalisks be harvested, the security officers tried to revolt, but crumpled after their leader was killed. The crystalisks angrily rose up in self-defense and slaughtered the mining team, resulting in the closing of the mine, and they've been hostile towards humans ever since.

Xenoblade Chronicles X: The Milsaadi are said to be silicon-based. They have metallic skin and their voices have a strange reverb to them, but they are definitely stated to not be mechanical. Given the Milsaadi's lack of relevance though, this ends up just being a trivia bit, not related to the plot at all.

Inara and Terminus from Paladins are members of a race of stone-skinned people called Stagala. Inara is a normal Stagala with the power to manipulate the earth, while Terminus is an undead warrior who fights using his massive axe and undead magic.

Gorons in The Legend of Zelda seem to be this. It's never outright stated, but they eat rocks and can take a lava bath without harm.

Web Comics

Schlock Mercenary: A halfway house between this and regular life: Sergeant Schlock, who is carbosilicate (i.e. based around both carbon and silicon). In a bit of realism, his biology and biochemistry is pretty clearly shown to be fundamentally different from traditional carbon-based life.

Vexxarr has two silicon-based species, both living in space. "Rock crabs" such as "Sid" (Silicon dioxide, get it?) are practically indestructible and feed off of radiation, while silicoid predators eat rock crabs and explode if fed cake. Both species are typically found roughly human-sized but grow to the size of asteroids.

SCP-229 ("Wire Weed"). Foundation scientists tentatively identified SCP-229 (a mass of wires and electrical cables) as a form of silicon-based life.

SCP-440 ("Sand-Based Ecology"). Several types of creatures are made up of SCP-440, which is an anomalous form of sand. The sand is made up of silica (silicon dioxide).

SCP-507 ("The Reluctant Dimension Hopper"). On one of SCP-507's trips he ended up in a dimension with living, moving crystals based on silicon. They're described as looking like crustaceans and may be connected to SCP-440, which also creates crustacean-like silicon-based life.

SCP-1107 ("A Signal"). A group of asteroid-like bodies in the Kuiper Belt and the Oort Cloud around the solar system contain unusually pure silicon and silicate based crystals. They absorb solar radiation and use the energy to generate radio signals. Analysis of the signals indicates that they're not only alive, but intelligent.

SCP-2978 ("Motherburg"). SCP-2978-B are a race of tiny silicon-based humanoids. They live in the silicon chips of computer motherboards and are experts at using and modifying computers.

SCP-3284-J ("Lava Sharks"). These are shark-like creatures that live in lava pools and can swim through them the way normal sharks swim through water. They don't need to eat but attack and consume living creatures anyway.

Invasion America has a genetically-engineered species called manglers. A baffled human scientist describes the discovered bones of one as a "silicon analog" rather than carbon-based.

This trope is invoked in the episode of The Simpsons when Homer encounters an alien.

Homer: I'd be happy to answer your questions about the alien. Any questions at all. Dr. Hibert: Is the alien carbon based or silicon based? Homer: Uh ... the second one. Silliphone.

Diamondhead and his expy, Chromastone, as well as Echo Echo in Ben 10. The difference with Echo Echo is it's species is not totally silicon: it's body is a suit made of silicon to hold its real body, which is a sonic wave.

In one episode of the Flash Gordon animated series, Ming the Merciless created a silicon based monster and unleashed it on the heroes. The heroes weapons had no effect on the creature because, in their words, they were shooting sand.

The aptly named Silicon Red, a folk singer resembling a bearded humanoid made out of stone and one of Bender's celebrity idols (asides Elzar and Calculon) from the seventh season Futurama episode "Forty Percent Leadbelly".

Real Life

Silicon is more abundant than carbon on the Earth's surface, and yet Earth life is almost exclusively carbon-based. Compared to carbon, silicon only presents an advantage in temperatures around 150°C, where silicon bonds would remain stable while their carbon counterparts break down. It is speculated that such silicon-based life would use siloxane backbones (alternating silicon-oxygen chain), as siloxanes can form a wide variety of polymers, and would probably use sulfuric acid as a solvent and respire fluorine gas. Such life would be prone to exploding on contact with an earth-like atmosphere.

Hypothetical sulfur and fluorosiloxane based-life would live in even hotter environments and, at the hotter end, use molten rock as a solvent.

While life on Earth is carbon-based in its molecular structure, one origin-of-life theory posits that organic molecules could've initially begun copying themselves on the surfaces of wet clays. As clays do contain silicon, this would make "silicon-based life" true in a literal sense.

Sponges use silicon in their skeletal structure while grass uses silicon as an abrasive to render itself less palatable as a food source and diatoms use silica to produce their frustules (hard outer coatings).

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